Traditionally, integrated circuits (IC) are built on or near the top surface of semiconductor chips. Electrical currents in the IC flow in and between circuit elements in parallel and close to the chip surface and certain locations in the surface areas are susceptible to stress from strong electric fields and high currents during the operation of the IC.
Recently, some circuit elements have been disposed away from the chip surface towards the bulk of the chip as a way to spread the current in order to reduce the resistance to current flow, and also to redirect the electric field away from the chip surface in order to increase the device operating voltage. As a result, trench structures are gaining popularity in power MOSFETs and rectifiers and transient voltage suppressing devices. This class of devices is commonly referred to as vertical devices or vertical ICs.
In some vertical ICs all trenches are of the same depth, such as D5VOLIB2DLP3, a 6 V, 6 A, 15 pF Zener TVS by Diodes Incorporated. In those devices, the trenches are defined with a single photo-mask and etched concurrently. In other vertical integrated circuits the trenches are of different depths, such as the MOSFET described in the U.S. Pat. No. 8,748,976 (the '976 patent). In the MOSFET disclosed in the '976 patent there are vertical RESURF trenches and gate trenches, which are of different depths and they are defined separately using a dedicated RESURF trench mask and a dedicated gate trench mask.